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Ultracompact low-loss coupler between strip and slot waveguides
KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP. KTH, School of Information and Communication Technology (ICT), Centres, Zhejiang-KTH Joint Research Center of Photonics, JORCEP.
KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP. KTH, School of Information and Communication Technology (ICT), Centres, Zhejiang-KTH Joint Research Center of Photonics, JORCEP.
KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP. KTH, School of Information and Communication Technology (ICT), Centres, Zhejiang-KTH Joint Research Center of Photonics, JORCEP.
KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP. KTH, School of Information and Communication Technology (ICT), Centres, Zhejiang-KTH Joint Research Center of Photonics, JORCEP.ORCID iD: 0000-0001-5967-2651
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2009 (English)In: Optics Letters, ISSN 0146-9592, E-ISSN 1539-4794, Vol. 34, no 10, 1498-1500 p.Article in journal (Refereed) Published
Abstract [en]

We present both theoretical and experimental results of an ultracompact waveguide coupler that is capable of highly efficient coupling of light from strip waveguides to slot waveguides, and vice versa. By optimizing the geometrical parameters, it is possible to achieve extremely low-loss coupling. A coupling efficiency of 97% has been obtained experimentally while keeping the overall size down to the range below 10 mu m. Further analysis shows that the proposed coupler has relatively high tolerance to fabrication errors and is wavelength insensitive. (C) 2009 Optical Society of America

Place, publisher, year, edition, pages
2009. Vol. 34, no 10, 1498-1500 p.
Keyword [en]
ring resonators, confining light, fibers
Identifiers
URN: urn:nbn:se:kth:diva-13074DOI: 10.1364/OL.34.001498ISI: 000267064500003Scopus ID: 2-s2.0-66349112883OAI: oai:DiVA.org:kth-13074DiVA: diva2:328289
Note
QC20100702Available from: 2010-07-02 Created: 2010-05-26 Last updated: 2013-11-19Bibliographically approved
In thesis
1. Investigation of New Concepts and Solutions for Silicon Nanophotonics
Open this publication in new window or tab >>Investigation of New Concepts and Solutions for Silicon Nanophotonics
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Nowadays, silicon photonics is a widely studied research topic. Its high-index-contrast and compatibility with the complementary metal-oxide-semiconductor technology make it a promising platform for low cost high density integration. Several general problems have been brought up, including the lack of silicon active devices, the difficulty of light coupling, the polarization dependence, etc. This thesis aims to give new attempts to novel solutions for some of these problems. Both theoretical modeling and experimental work have been done.

Several numerical methods are reviewed first. The semi-vectorial finite-difference mode solver in cylindrical coordinate system is developed and it is mainly used for calculating the eigenmodes of the waveguide structures employed in this thesis. The finite-difference time-domain method and beam propagation method are also used to analyze the light propagation in complex structures.

The fabrication and characterization technologies are studied. The fabrication is mainly based on clean room facilities, including plasma assisted film deposition, electron beam lithography and dry etching. The vertical coupling system is mainly used for characterization in this thesis. Compared with conventional butt-coupling system, it can provide much higher coupling efficiency and larger alignment tolerance.

Two novel couplers related to silicon photonic wires are studied. In order to improve the coupling efficiency of a grating coupler, a nonuniform grating is theoretically designed to maximize the overlap between the radiated light profile and the optical fiber mode. Over 60% coupling efficiency is obtained experimentally. Another coupler facilitating the light coupling between silicon photonic wires and slot waveguides is demonstrated, both theoretically and experimentally. Almost lossless coupling is achieved in experiments.

Two approaches are studied to realize polarization insensitive devices based on silicon photonic wires. The first one is the use of a sandwich waveguide structure to eliminate the polarization dependent wavelength of a microring resonator. By optimizing the multilayer structure, we successfully eliminate the large birefringence in an ultrasmall ring resonator. Another approach is to use polarization diversity scheme. Two key components of the scheme are studied. An efficient polarization beam splitter based on a one-dimensional grating coupler is theoretically designed and experimentally demonstrated. This polarization beam splitter can also serve as an efficient light coupler between silicon-on-insulator waveguides and optical fibers. Over 50% coupling efficiency for both polarizations and -20dB extinction ratio between them are experimentally obtained. A compact polarization rotator based on silicon photonic wire is theoretically analyzed. 100% polarization conversion is achievable and the fabrication tolerance is relatively large by using a compensation method.

A novel integration platform based on nano-epitaxial lateral overgrowth technology is investigated to realize monolithic integration of III-V materials on silicon. A silica mask is used to block the threading dislocations from the InP seed layer on silicon. Technologies such as hydride vapor phase epitaxy and chemical-mechanical polishing are developed. A thin dislocation free InP layer on silicon is obtained experimentally.

Place, publisher, year, edition, pages
Stockholm: KTH, 2010. xii, 79 p.
Series
Trita-ICT/MAP AVH, ISSN 1653-7610 ; 2010:4
Keyword
Planar integrated circuit, silicon photonics, slot waveguide, finite-difference time-domain, waveguide grating coupler, ring resonator, polarization diversity scheme, polarization beam splitter, polarization rotator, hybrid silicon laser, epitaxial lateral overgrowth, chemical-mechanical polishing.
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:kth:diva-13029 (URN)
Public defence
2010-06-11, C1, Electrum 1, Isafjordsgatan 26, Kista, 13:47 (English)
Opponent
Supervisors
Note
QC20100705Available from: 2010-05-26 Created: 2010-05-24 Last updated: 2011-08-30Bibliographically approved
2. Study on electroabsorption modulators and grating couplers for optical interconnects
Open this publication in new window or tab >>Study on electroabsorption modulators and grating couplers for optical interconnects
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Decades of efforts have pushed the replacement of electrical interconnects by optical links to the interconnects between computers, racks and circuit boards. It may be expected that optical solutions will further be used for inter-chip and intra-chip interconnects with potential benefits in bandwidth, capacity, delay, power consumption and crosstalk. Silicon integration is emerging to be the best candidate nowadays due to not only the dominant status of silicon in microelectronics but also the great advantages brought to the photonic integrated circuits (PICs). Regarding the recent breakthroughs concerning active devices on silicon substrate, the question left is no longer the feasibility of the optical interconnects based on silicon but the competitiveness of the silicon device compared with other alternatives.

This thesis focuses on the study of two key components for the optical interconnects, both especially designed and fabricated for silicon platform. One is a high speed electroabsorption modulator (EAM), realized by transferring an InP-based segmented design to the hybrid silicon evanescent platform. The purpose here is to increase the speed of the silicon PICs to over 50  Gb/s or more. The other one is a high performance grating coupler, with the purpose to improve the optical interface between the silicon PICs and the outside fiber-based communication system.

An general approach based on the transmission line analysis has been developed to evaluate the modulation response of an EAM with a lumped, traveling-wave, segmented or capacitively-loaded configuration. A genetic algorithm is used to optimize its configuration. This method has been applied to the design of the EAMs on hybrid silicon evanescent platform. Based on the comparison of various electrode design, segmented configuration is adopted for the target of a bandwidth over 40 GHz with as low as possible voltage and high extinction ratio.

In addition to the common periodic analysis, the grating coupler is analyzed by the antenna theory assisted with an improved volume-current method, where the directionality of a grating coupler can be obtained analytically. In order to improve the performance of the grating coupler, a direct way is to address its shortcoming by e.g. increasing the coupling efficiency. For this reason, a nonuniform grating coupler with apodized grooves has been developed with a coupling efficiency of 64%, nearly a double of a standard one. Another way is to add more functionalities to the grating coupler. To do this, a polarization beam splitter (PBS) based on a bidirectional grating coupler has been proposed and experimentally demonstrated. An extinction ratio of around -20 dB, as well as a maximum coupling efficiency of over 50% for both polarizations, is achieved by such a PBS with a Bragg reflector underneath.

Place, publisher, year, edition, pages
Stockholm: KTH, 2010. xii, 80 p.
Series
Trita-ICT/MAP AVH, ISSN 1653-7610
Keyword
Photonic integrated circuit, optical interconnect, transmission line, traveling-wave, electro-absorption modulator, hybrid silicon evanescent platform, grating coupler, vertical coupling scheme, optical antenna, lag effect, polarization beam splitter
National Category
Telecommunications
Research subject
SRA - ICT
Identifiers
urn:nbn:se:kth:diva-24178 (URN)
Public defence
2010-09-27, sal 438, Isafjordsgatan 39, Kista, Stockholm, Forum, KTH, 10:00 (English)
Opponent
Supervisors
Note
QC 20100906Available from: 2010-09-06 Created: 2010-08-19 Last updated: 2011-08-30Bibliographically approved

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Wosinski, LechHe, Sailing

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